1. Field of the Invention
The present invention relates to a method of producing a steel sheet plated with a Zn-Mg alloy. The invention also is concerned with such a steel sheet, which is superior both in plating adhesion and corrosion resistance and which is suitable for use in automobiles, household electric appliances, architecture and so forth.
2. Description of the Related Art
Hitherto, steel sheets have been widely used in automobiles, household electric appliances, architecture and so forth. Conventional steel sheets tend to become rusty under normal use. To avoid this problem, plated steel sheets are finding widespread use. Typically, Zn plating of steel sheets has been adopted for a long time. In recent years, however, various Zn type plating alloys have been developed and used to cope with the demand for enhancement of anti-rust performance of steel sheets.
Zn-alloy plating is broadly sorted into two types: namely, hot-dip plating with Zn-Fe alloy or Zn-Al.lambda. alloy, and electroplating with Zn-Ni alloy or Zn-Fe alloy. These plating methods are selected according to the uses but do not provide satisfactory rust prevention. On the other hand, there is a trend for diversified demands of users. In order to meet such demands while attaining sufficient rust prevention, various attempts and studies are being conducted to develop novel plating techniques.
Plating with an alloy formed by adding Mg to Zn, as one of such attempts, is now calling for attention. Mg is an element which effectively enhances the rust prevention effect inherently possessed by Zn, and various Zn-Mg alloys for plating, as well as methods of producing such alloys, have been proposed.
Hot-dip plating was considered first as a method of plating with Zn-Mg alloy, as well as a production method. Hot-dip plating techniques using Zn-Mg alloys are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 56-96036, 56-123359, 56-152953 and 56-152956. Hot-dip plating with a Zn-Mg alloy, however, suffers from the following problem. Namely, since Mg has a melting point of 650.degree. C. much higher than that of Zn which is 419.degree. C., Mg can be added to Zn plating bath only in a very small quantity, e.g., less than 1 wt%. In addition, the high temperature of the plating bath adversely affects the properties of the steel sheet to be plated, causing problems such as impairment of workability of the steel sheet.
Products plated with Zn-Mg alloys by physical vapor deposition are shown in Japanese Patent Laid-Open Publication Nos. 64-17851, 64-17852 and 64-17853. Physical vapor deposition, however, requires a heat source of large output power for the purpose of vaporizing Mg and Zn, which undesirably raises the cost of the production equipment. In addition, this plating method cannot provide high plating adhesion and cannot provide a fine uniform plating layer.
Electroplating with a Zn-Mg alloy also is difficult to conduct when an ordinary aqueous solution is used, due to the excessive difference in electrochemical potential between Zn and Mg. Although a plating bath containing a fluoride is disclosed in Japanese patent Laid-Open Publication No. 58-144492, this plating bath cannot contain Mg in excess of 1 wt %.
Thus, it has been impossible to obtain a steel sheet which is plated with a Zn-Mg alloy and which is superior both in plating adhesion and corrosion resistance.